1. Field of the Invention
The present invention relates to surface acoustic wave (hereinafter abbreviated as SAW) duplexers having an impedance matching circuit at a coupling section between a plurality of SAW filters and an antenna terminal, and communication apparatuses including such SAW duplexers.
2. Description of the Related Art
Recently, multiple-band portable telephones including two or more communication systems have been examined as highly functional mobile communication apparatuses such as portable terminals. Since a plurality of communication systems are integrated into one portable terminal, its components need to be compact and highly functional. Therefore, the use of SAW filters, which is effective for compactness, has been examined for dual filters in which two filters having different pass-band frequencies are integrated, and for duplexers, such as antenna duplexers.
Japanese Unexamined Patent Application Publication No. 02-69012 discloses such a duplexer in which a SAW-filter chip is inserted into a sealable case for maintaining airtightness. This publication describes an advantage that easier handling and more compactness are expected than with a conventional dielectric filter because the sealable case provides a shield effect and the sealable case of the SAW filter does not need to be soldered to a substrate.
In addition, since the duplexer described in this publication uses an inductor as a branching circuit functioning as an impedance matching circuit, and the inductor has a shorter line length than a conventional stripline defined by a distributed constant line, an area occupied by the branching circuit is made smaller. Therefore, in addition to the above-described effect, this can make the duplexer more compact. In this publication, the inductor is mounted on the substrate rear surface to make the duplexer compact.
Japanese Unexamined Patent Application Publication No. 05-167388 discloses a duplexer which uses a band-pass filter including a plurality of SAW resonators. The publication describes as one structure of a duplexer according to its second embodiment that an inductor is inserted between a connection point of transmission and receiving signals and the ground as an impedance matching circuit.
In addition, as another duplexer, Japanese Unexamined Patent Application Publication No. 05-167389 discloses a duplexer which can be made as compact as possible while maintaining a satisfactory isolation. The publication discloses in its sixth embodiment that three external signal terminals are disposed at three sides, and grounded portions are disposed at both sides of each signal terminal to prevent signals from leaking.
In a duplexer using the above-described conventional SAW filter, however, since the duplexer has been made more compact, and the distance between terminals such as an antenna terminal, a transmission terminal, and a receiving terminal, has been made short, a problem is that a sufficient isolation cannot be obtained between terminals.
In Japanese Unexamined Patent Application Publication No. 02-69012, each signal terminal is disposed on the same side. In contrast, in Japanese Unexamined Patent Application Publication No. 05-167389, signal terminals are disposed at three sides, and grounded portions are disposed at both sides of each signal terminal to prevent signals from leaking among the signal terminals.
In general, a matching circuit is required at an antenna coupling section of a SAW duplexer. As shown in Japanese Unexamined Patent Application Publication No. 02-69012 and Japanese Unexamined Patent Application Publication No. 05-167388, a parallel inductor is used in some cases. When an inductor is connected to an antenna terminal as a matching circuit, and the other end is connected to the ground as shown in FIG. 1B of Japanese Unexamined Patent Application Publication No. 02-69012, it is convenient for wiring purposes that a ground electrode is disposed at the other side of the antenna terminal.
With the above-described arrangement method, however, a sufficient isolation can not be obtained between the antenna terminal and other terminals, and between the transmission terminal and the receiving terminal.
In order to overcome the problems described above, preferred embodiments of the present invention provide a SAW duplexer in which an impedance matching circuit is provided at an antenna coupling section and excellent isolation characteristics are provided, and also provide a communication apparatus including such a novel SAW filter.
According to a preferred embodiment of the present invention, a surface acoustic wave duplexer includes a branching section provided on a substrate, the branching section including a plurality of surface acoustic wave filters arranged to perform a branching function for transmission and reception, an antenna terminal, a transmission terminal, a receiving terminal, and a ground terminal each connected to the branching section and provided at a peripheral portion of the substrate, and a stripline having a matching function being arranged such that one end thereof is connected to the antenna terminal and the other end thereof is grounded to at least one side other than a side opposite to a side where the antenna terminal is located.
According to the above-described unique structure, since the plurality of SAW filters are used, the branching function for transmission and receiving is provided. In addition, since the stripline having the matching function and connected to the antenna terminal is provided, matching of an input and output impedance at the antenna terminal is achieved. Therefore, a transmission characteristic for transmission and reception to and from the antenna terminal is greatly improved.
Further, in the above-described unique structure, since the stripline is arranged such that it is grounded to at least one side other than a side opposite to a side where the antenna terminal is located, the deterioration of attenuation from the antenna terminal to the receiving terminal is prevented and minimized, and the deterioration of attenuation in an isolation characteristic from the transmission terminal to the receiving terminal is also prevented and minimized.
The above-described SAW duplexer may be configured such that the substrate is a multi-layer substrate having a plurality of conductive patterns for defining an electric-element circuit and a connecting line, in a thickness direction, and the stripline is part of the conductive patterns, and connected to a grounding conductive pattern provided on the same layer as the conductive patterns.
According to the above-described structure, since the substrate is preferably a multi-layer substrate, compactness is provided and wiring to each terminal can be more flexibly installed. Therefore, interference between terminals is prevented and minimized.
The above-described SAW duplexer may be configured such that the stripline is grounded to the side where the antenna terminal is located.
According to the above-described structure, furthermore, the deterioration of attenuation from the antenna terminal to the receiving terminal is more effectively prevented and minimized, and the deterioration of attenuation in an isolation characteristic from the transmission terminal to the receiving terminal is also more effectively prevented and minimized.
The above-described SAW duplexer may be configured such that the stripline is grounded in an area that is different from an area including the receiving terminal, among two areas into which the substrate is divided by an imaginary line connecting the antenna terminal and the transmission terminal.
According to the above-described structure, further, since the stripline is grounded in the area that is different from the area which has the receiving terminal, among the two areas into which the substrate is divided by an imaginary line connecting the antenna terminal and the transmission terminal, isolation from the antenna terminal to the receiving terminal is more positively maintained at a satisfactory level. Therefore, the above-described structure provides compactness and more effectively achieves an excellent attenuation characteristic.
The above-described SAW duplexer may be configured such that the transmission terminal and the receiving terminal are provided on both corner sections along a first side of the substrate, respectively, and the antenna terminal is provided in a center section of a second side opposite to the first side.
According to the above-described structure, since mutual interference between any two terminals of the antenna terminal, the transmission terminal, and the receiving terminal is prevented and minimized, a satisfactory isolation is more positively obtained between any two of the above terminals.
The above-described SAW duplexer may be configured such that a ground terminal is disposed between any two terminals of the antenna terminal, the transmission terminal, and the receiving terminal.
According to the above-described structure, since mutual interference between any two terminals of the antenna terminal, the transmission terminal, and the receiving terminal is more reliably prevented and minimized by the ground terminal disposed between any two of the terminals, a satisfactory isolation is further more reliably achieved between any two of the above terminals.
The above-described SAW duplexer may be configured such that the stripline has a coil shape. According to the above-described structure, since the stripline preferably has a coil shape, a much smaller space is required for this duplexer because of the great compactness achieved.
In addition, since the coil-shaped stripline also defines an inductor element in the above-described structure, it functions not only as a matching element but also as an element for suppressing harmonic signals. Therefore, the emission of useless harmonic signals from a communication apparatus, such as a portable terminal, is effectively prevented and minimized.
In yet another preferred embodiment of the present invention, a communication apparatus includes one of the above-described surface acoustic wave duplexers.
According to the above-described structure, since the communication apparatus has a compact SAW duplexer having a satisfactory isolation between any two terminals of the antenna terminal, the transmission terminal, and the receiving terminal, compactness is provided and communication characteristics, such as a transfer characteristic, is greatly improved.
As described above, a SAW duplexer according to preferred embodiments of the present invention has a structure in which a branching section having a plurality of SAW filters is provided on a substrate, and a stripline connected to an antenna terminal of the branching section is grounded to at least a side other than the side opposite the side where the antenna terminal is disposed.
Therefore, the above-described structure achieves matching of an antenna terminal connected to SAW filters by use of the stripline while providing compactness, and also provides an excellent attenuation characteristic and an excellent isolation characteristic.
As described above, a communication apparatus according to a preferred embodiment of the present invention has the above-described SAW duplexer. Therefore, since the SAW duplexer is compact and has multiple functions and an excellent transmission characteristic, the structure of the communication apparatus provides an excellent transmission and reception function as well as compactness.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.